Arc heater method for producing metal oxides

ABSTRACT

An arc heater method and system for producing a high energy source characterized by a housing forming a mixing compartment, a plurality of arc heaters radially mounted on the housing and communicating with the mixing compartment, a reactor communicating with the downstream end of the mixing compartment with a reduced opening therebetween, inlet means at the downstream end of the compartment for introducing an oxidizable metal compound into the reactor, and means within the mixing compartment for flowing superheated gases from the arc heaters at the upstream end of the compartment and into the reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a mixing chamber for use with electric archeaters for damping flow perturbations and vortices in heated gasstreams to produce smooth flowing gas streams.

2. Description of the Prior Art

Many industrial processes require large volume streams of gaseousreactants to be heated to high temperatures in order to supply thethermal demands of particular processes. In general, it is desirable tosupply the required energy without contaminating the reactant stream,and thus elaborate and generally complex heat exchange devices arefrequently used. A typical example is a process for production oftitanium dioxide pigment. For this endothermic reaction, titaniumtetrachloride (TiCl₄) is heated to a temperature of about 1000° C. andcombined with oxygen which is preheated to about 1200° C. The oxygen isheated in an externally fired heat exchanger to the maximum achievabletemperature with state of the art materials. Inconel is typically used.Titanium tetrachloride, being corrosive at elevated temperatures, cannotbe contained in conventional metals, and requires a very complex anddelicate externally fired serpentine heat exchanger in which the TiCl₄is contained in fused quartz tubing. Variations in flow and resultantpressure drop in the downstream reactor and coolant sections oftenproduce acoustic waves which cause vibrations in the delicate quartzserpentine. Rupture occurs when the resonant frequency is reached,resulting in expensive maintenance and loss of production.

The titania industry has attempted many process variations to circumventthe problems, but each has significant disadvantages. Fuel oil has beenadmixed with the hot oxygen, thus reducing the temperature requirementfor the TiCl₄ to a level suitable for conventional materials. Fuelcombustion increases the mass requirement for oxygen substantially, andproduces copious quantities of HCl upon reaction with the TiCl₄. The HClis very corrosive to the downstream materials of construction andfurthermore, it cannot be recycled for subsequent reuse.

Another process employs the combustion of carbon monoxide to provide thesuperheat energy, but it too suffers from increased costs of both oxygenand carbon monoxide. In another process, the oxygen is heated totemperatures of about 2000° C. by passing it through the discharge in aD.C. electric arc heater. This circumvents the need for the delicatequartz superheater for the TiCl₄ as the heat of reaction enters theprocess by superheat of the oxygen. The disadvantages of that processare limited production volume, employment of an expensive D.C. powersystem to supply the arc heater, and the geometery not being suitablefor conversion of existing gas fired processes.

SUMMARY OF THE INVENTION

In accordance with this invention, it has been found that thedisadvantages of some prior art procedures may be overcome by providingan arc heater system as a high energy heat source for producing metaloxides comprising a housing forming a mixing compartment with aplurality of arc heaters mounted substantially radially thereon at theupstream end. Each arc heater comprises a downstream electrode adjacentto the housing and an upstream electrode remote therefrom with a gaptherebetween through which oxygen is introduced into the system to forman elongated arc heated oxygen stream. A reactor communicates with thedownstream end of the housing. The housing comprises a refractory liningand includes axially extending vanes for directing the flow of gaseslongitudinally and devoid of vortices. The housing walls comprisecooling means whereby additional incoming oxygen is preliminarily passedthrough the cooling means and subsequently into the arc heated oxygenstream. Nozzle means are also provided in the mixing chamber forinjecting abrasive particles longitudinally into the chamber toward thereactor for preventing the accumulation of reaction materials on thewalls of the compartment and reactor. Conduit means between the mixingcompartment and the reactor of a dimension less than those of thecompartment are provided. Finally, inlet means for introducing anoxidizable metal compound into the reactor are provided at the upstreamend thereof.

The advantage of this arc heater system is to provide superheated gasessuitable for the production of clean oxides such as titania.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an arc heater system for producing ahigh energy heat source;

FIG. 2 is an end view taken on the line II--II of FIG. 1;

FIG. 3 is an enlarged sectional view of a portion of the arc heatersystem shown in FIG. 1;

FIG. 4 is a vertical sectional view taken on the line IV--IV of FIG. 3;

FIG. 5 is a vertical sectional view of another embodiment of the archeater system;

FIGS. 6, 7, and 8 are enlarged sectional views of other embodiments ofthe arc heater system; and

FIG. 9 is a graph showing operating parameters of the arc heater systemfor titania production.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of this invention provides for the production of metaloxides by employment of high energy heat and comprises the steps ofproviding a polyphase arc heater system characterized by a number of archeaters having arc chambers communicating with an elongated centralmixing compartment; striking an electric arc in an axial gap betweengenerally hollow cylindrical electrodes spaced along a common axis thatform the arc chamber in each of the inner communicating arc heaterchambers; directing oxygen through the gap into the arc chamber tothereby form an elongated arc heated oxygen stream; projecting theturbulent arc heated oxygen stream generally radially into the centralmixing compartment; introducing a turbulent arc heated oxygen streamthrough the longitudinal channel means in the central mixing compartmentto straighten the flow direction of the stream; introducing into the archeated oxygen stream a halide of a metal selected from the groupconsisting of magnesium, aluminum, silicon, titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, andtungsten, to form reaction products including an oxide of the metal anda halogen gas; directing particles of abrasive material longitudinallythrough the central mixing compartment and onto the downstream surfacesto prevent deposition of the reaction products thereon; and passingoxygen through channel means in the walls of the central mixingcompartment in heat exchange relationship therewith and then passing thepreheated oxygen into the elongated arc heated oxygen stream.

The process of this invention is preferably performed in an arc heatersystem indicated in the drawings. As shown in FIGS. 1 and 2, the archeater system comprises a plurality, preferably three, arc heaters 11,13, 15, a mixer 17, and a reactor 19. The embodiment of the system whichovercomes the disadvantages of prior systems, has high productioncapability and employs a three phase A.C. power system comprising thearc heaters 11, 13, 15, which are three self-stabilizing electric archeaters which supply the reaction energy by superheating oxygen. Eacharc heater 11, 13, 15 is connected line-to-ground in a wye-connectedthree phase system. The arc heater 11 is typical of the arc heaters 11,13, and 15 and is shown schematically in FIG. 3.

The arc heater 11 may be operated at a temperature ranging from about1650° C. to about 5500° C. A suitable arc heater for use herein is thatdisclosed in U.S. Pat. No. 3,832,519, issued Aug. 27, 1974, entitled"Arc Heater With Internal Fluid and Electrical Duct and Quick DisconnectFacility" of which the inventors are Charles B. Wolf, Maurice G. Fey,and Frederick A. Azinger, Jr. Because of the full disclosure in thatpatent, the description of the arc heater 11 is limited herein to thebasic structure in operation. The arc heater 11 is a single phase,self-stabilizing A.C. device capable of power levels up to about 3500kilowatts or up to 10,000 kilowatts for a three phase plantinstallation. For the practice of this invention, it is preferred thatthree arc heaters be provided, one for each of the three phases of theA.C. power supply.

As shown in FIG. 3, the arc heater 11 includes two annular copperelectrodes 21, 23 which are spaced by a gap 25 about 1 millimeter wideto accommodate the line frequency power source of 4 kV. An arc isinitiated in the space for gas 25, and incoming feed stock gas, such asoxygen, indicated by the arrow 29, is introduced through the gap into anarc chamber 31 within the interior of the arc heater 11. The arc 27rotates at a speed of about 60,000 rpm by interaction between the arccurrent (several thousand amps A.C.) with a D.C. magnetic field set upby internally mounted solenoid coils 33, 35. The velocities generatedyield a very high operating efficiency for equipment of this type. Exitenthalpies ranging from, for example, up to about 15,000 BTU per poundare easily obtained at good thermal efficiencies at the downstream exitend 37 of the arc heater.

In operation, the arc heaters 11, 13, 15, provide an elongated arcstream 27a which extends through the exit end 37 of the arc heater intothe mixer 17. The arc heaters 11, 13, 15 are physically located at 120°increments at the upstream end of the mixer 17 and fire the elongatedarc stream 27a radially inward into a central mixing compartmentcontained within a refractory lining 39. This serves to dampen thetemperature perturbations of the three alternating power waves. Vorticesare removed from this stream by passing it through longitudinal passages41 (FIG. 4) formed by longitudinally extending vanes 43 which vanesextend radially from the center of the compartment to the refractorylining 39. Thus, the vanes 43 serve as an internal flow straightener forthe oxygen gas stream flowing downstream (to the right as viewed in FIG.3) at the downstream end of the mixer 17, where a passage 45 having adiameter less than that of the mixing chamber 38 is provided toaccelerate the axially flowing gas stream to the required velocity asthe gas stream enters the reactor 19. There the oxygen gas streamcontacts a radially directed stream of a metal compound, such astitanium tetrachloride, and reacts to form a metal oxide, namely,titanium dioxide.

Structurally, the walls of the mixing chamber are made of a suitablehigh temperature material, such as stabilized zirconium oxide, withwhich is operated at internal surface temperatures of up to about 2500°C. It is recognized that other materials may also be used, such asthorium oxide, magnesium oxide, zirconium diboride, magnesium chromate,and lanthanum chromate. Heat lost by conduction through the walls ispartially recovered by incoming oxygen which is introduced at an inlet46 which communicates with interconnecting passage means 47, 49 thatlead to oxygen nozzle 51 where the heated oxygen is injected into thearc stream 27a. Thus, the preheated oxygen stream is internallymanifolded to the process stream at the exit flanges of each of the archeaters. The relative oxygen flow entering the two locations at the gap25 and the nozzle 51 is adjusted for temperature control of the oxygenstream.

The reactor 19 is a tubular container, the upstream end of which isattached to the downstream end of the mixer 17. Oxidizable compounds ofmetals, such as for example, titanium, zirconium, hafnium, magnesium andaluminum, are introduced into the reactor 19 through an inlet 53 betweenthe mixer 17 and the reactor 19 (FIGS. 1 and 3). Upon entering thereactor chamber, the oxidizable metal compound, such as titaniumtetrachloride, reacts with the hot oxygen gases entering the reactor inaccordance with the following formula:

    TiCl.sub.4 + O.sub.2 → TiO.sub.2 + 2Cl.sub.2

In the reactor 19, the reaction temperature varies from about 1600° to2300° C. The reaction products including titanium dioxide and chlorinegas move from the reactor into a cyclone separator 55 where the titaniumdioxide, being very fine particles, exits the system with the chlorinegas.

As shown in FIGS. 3 and 4, particles of an abrasive material such assand are introduced through a conduit 57 which is axially disposed inthe mixer 17. The inner right end of the conduit is directed toward thepassage 45 so that the particles of sand are projected against thesurfaces of the passage and of the reactor 19 in order to keep thesurfaces free from accumulation of reaction materials and products andthereby maintain an efficient reaction procedure. The sand is ultimatelyseparated from the reaction products (FIG. 1) in the cyclone separator55.

Another embodiment of the invention is shown in FIG. 5 in which partswith reference numbers similar to those of the system of FIGS. 1, 2 and3 are similarly identified. More particularly, in FIG. 5, one arc heater11 is shown though it is understood that two additional arc heaterssimilar to the arc heaters 13 and 15 (FIG. 2) are included. The archeaters direct hot oxygen streams into a mixer 59 having a refractorylining 61 similar to the lining 39. A mixing chamber 63 formed by thelining 61 is filled with refractory bodies 65, such as bricks whichcontribute to the gas mixing in the chamber.

More particularly, the three arc heaters 11, 13, and 15 operating atdifferent cycles of the A.C. power source operate at slightly differenttemperatures so that oxygen introduced into the mixer 59 by each archeater has a temperature different from the oxygen introduced in theother two arc heaters. By providing the bricks 65 in the mixing chamber63, the gases become thoroughly mixed and achieve a uniform temperatureas mixed gases rise through the top of the mixer 59. As the gas leavesthe mixer 59, it enters a conduit 67 of a diameter less than that of themixing chamber 63 from where it is directed into the reactor 19 in amanner similar to that of the system of FIG. 1. An inlet 69 is providedfor injection of particles of abrasive material such as sand which isnecessary to prevent reaction products from accumulating on the walls ofthe reactor 19. Manifestly, the inlet 53 between the conduit 67 and thereactor 19 introduces an oxidizable compound of a metal which isconverted to the metal oxide in the reactor 19. The reaction products,such as titanium dioxide and chlorine gas, upon leaving the reactor 19are directed into a cyclone separator similar to that shown in FIG. 1 inorder to separate the reaction products from the sand.

Another embodiment of the invention is shown in FIG. 6 in which similarparts are provided with similar reference numbers as those of priorembodiments. In this embodiment, the mixture chamber 38 is devoid oflongitudinal vanes 43 as provided in the first embodiment (FIG. 3). Inaddition, the downstream end of the chamber communicates directly withthe reactor with the omission of a reduced passage 45 as shown in FIG.3. Finally, a conduit 71 for the injection of particles of an abrasivematerial such as sand extends only slightly into the upstream end of themixture chamber 38 thereby enabling the projection of sand onto thesurface of the mixing chamber 38.

In another embodiment of the invention shown in FIG. 7, the mixingchamber 38 is substantially shorter than those of the previousembodiments and the inlet 53 for the introduction of oxidizablecompounds of a metal such as titanium tetrachloride as an inner inlet 73upstream of the inlet 53, whereby the metal compound is in immediatecontact with the heated oxygen leaving the mixing compartment 38.Finally the reactor 19 is provided with a water jacket 75.

Finally, another embodiment is shown in FIG. 8 in which heated oxygenissuing from the arc heaters 11, 13, 15 is delivered into an annularplenum chamber 77 from where it exits through outlet means 79 directlyinto the reactor 19. An oxidizable compound of a metal such as titaniumtetrachloride is introduced into the reactor upstream of the outletmeans 79 for oxygen through a conduit 81 surrounding the conduit 71 forthe introduction of particles of abrasive material such as sand.

The operating parameters of the gas superheater of this invention forthe production of titania are shown in FIG. 9. For an expectedproduction volume of 60 tons per day of TiO₂, an arc heater power of 900kW will be required at 85% thermal efficiency and for a stoichiometricoxygen excess of 10%. Manifestly, the gas superheater of this inventionmay be used for heating gases for a wide range of purposes and notsimply limited to heating oxygen for the production of pure oxides suchas titania. For example, steam may be heated for pyrolysis ofhydrocarbons to produce products such as ethylene and propylene.Nitrogen could be heated for the production of nitrides such as boronnitride.

What is claimed is:
 1. In a process for producing metal oxides involvingproviding a polyphase arc heater system characterized by a plurality ofarc heaters having arc chambers communicating with an elongated centralmixing compartment; striking an electric arc in an axial gap betweengenerally hollow, cylindrical electrodes spaced along a common axis thatform the arc chamber in each of the intercommunicating arc heaterchambers; causing an arc to rotate within the electrodes; directingoxygen through the gap into the arc chamber to thereby form an elongatedarc and heated oxygen stream at a temperature of from about 1600° toabout 2300° C.; projecting the turbulent arc-heated oxygen streamgenerally radially into the central mixing compartment; the improvementcomprising the steps of flowing the turbulent arc-heated oxygen streamthrough longitudinal channel means in the central mixing compartment tostraighten the flow direction of the stream; introducing into thearc-heated oxygen stream a halide of a metal selected from the groupconsisting of Ti, Zr, Hf, Mg, and Al to form reaction products includingan oxide of the metal and a halogen gas; and separating the reactionproducts.
 2. The process of claim 1 wherein particles of abrasivematerial are directed longitudinally into the central mixing compartmentand onto downstream surfaces to prevent deposition of reaction materialsthereon.
 3. The process of claim 1 wherein additional oxygen is passedthrough channel means in the walls of the control mixing compartment inheat exchange relationship therewith and then passing the preheatedoxygen into the elongated arc-heated oxygen stream.